Encapsulation Key to In Vivo Imaging

Recent research indicates that quantum dots could someday replace fluorescent markers, such as organic dyes and fluorescent proteins, in some biological imaging applications. To compete, though, these nanometer-scale semiconductor crystallites must simultaneously provide efficient fluorescence, colloidal stability and low nonspecific adsorption in an aqueous solution -- which so far has not been easy to do.

The solution could lie in the encapsulation of quantum dots in phospholipid block-copolymer micelles, said researcher Benoit Dubertret of the Laboratoire d'Optique Physique at ESPCI in Paris.

The draw of quantum dots lies in their ability to act as broadly tunable nano-emitters that are excitable with a single light source. To make them water soluble, scientists must work around any water-phobic outer coating produced during their synthesis. Possible solutions have involved replacing those coatings with more amenable ones. According to Dubertret and his colleague David J. Norris of the University of Minnesota in Minneapolis, these techniques have had some success in applications such as providing reagents for fluoro-immunoassays, but limited success in applications where quantum dots are used for fluorescence in situ hybridization or as markers for molecular recognition on cell surfaces.

Working with colleagues at Rockefeller University in New York and at NEC Research Institute in Princeton, N.J., Dubertret and Norris devised an encapsulation technique that allows nanocrystal micelles, when conjugated to DNA, to act as in vitro fluorescent probes. They also demonstrated in vivo imaging. In one experiment where the quantum-dot micelles were injected into frog embryos, the researchers followed the fluorescence through the tadpole stage (see figure). During imaging, the biomarkers were reportedly stable, minimally toxic to the cell and slow to photobleach.

When nanocrystal micelles were injected into frog embryos, researchers were able to follow the fluorescence from blastomere through tadpole stage (B-E, fluorescence and transmission images are superimposed).
Also of note, the scientists encapsulated individual ZnS-coated CdSe quantum dots within micelles without modifying their surfaces. In addition, Dubertret and colleagues report that the phospholipid micelles used have the advantage of being regular in size, shape and structure, and that the micelle-forming hydrophilic polymer-grated lipids are comparable to naturally occurring carriers such as lipoproteins and viruses.

Although commercialization of the encapsulation technique could take place relatively soon, research efforts are ongoing. "We have demonstrated a novel method to make the quantum-dot water soluble and biocompatible when injected in Xenopus [frog] embryos," Dubertret added. "We now need to test this new material in various systems both in vivo and in vitro. The block-copolymers that we used may need to be modified to yield even better stability in vivo."

Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.